Superconducting wires applied to superconducting magnets employed for strong magnetic fields and to superconductors employed in nuclear fusion reactors are subject to electromagnetic force during conduction, and large strains are imparted to the superconducting wires during cooling. In particular, a superconducting wire with a twisted wire structure, in which plural element wires are twisted together, is sometimes imparted with not only tensile stress in the element wire length direction, but also with a complicated strain such as strain in the compression direction and strain in bending directions. There is a need for achieving higher strength of a superconducting wire in order to suppress a drop in superconducting characteristics of the superconducting wire even in a state imparted with such strain.
In order to achieve higher strength of a compound superconducting wire, there are a method of compositing a high strength material within a superconducting wire, and a method of assembling a superconducting wire and a member consisting of a high strength material.
As a method of compositing a high strength material within a superconducting wire, there is a method, as described in Japanese Patent Application Laid-Open (JP-A) No. 2001-57118, in which, out of compound superconducting wires, an Nb3Sn superconducting wire containing an Nb3Sn superconductor is formed into a reinforced Nb3Sn superconducting wire by using a reinforcement consisting of a CuNb alloy formed by an in-situ method. Moreover, as described in JP-A No. 2008-300337, there is a known method of forming a high strength superconducting wire by using a method of compositing a reinforcement element wire, in which Ti as a rod shaped reinforcement member is embedded in Cu, and an internal diffusion method element wire in an Nb3Sn superconducting wire formed by an internal diffusion method.
As a method of assembling a superconducting wire and a member consisting of a high strength material, there is a known method, as described in JP-A No. H05-217433, in which a superconducting wire and stainless steel are assembled together.
However, the superconducting wires including the reinforcement consisting of a CuNb alloy formed with Cu by an in-situ method as in JP-A No. 2001-57118, have the problems of lacking uniformity in the length direction, being liable to wire breakage during processing, and the residual resistivity ratio of the reinforcement portion becoming smaller. Thus, in cases in which attempts are made to employ a superconducting wire using a reinforcement consisting of CuNb alloy formed by an in-situ method in a superconducting magnet employed for strong magnetic fields, there is a need to increase the amount of copper wire composited with the superconducting wire in order to secure conduction stability, and as a result there is the problem that the current density per superconducting wire falls. Moreover, in cases in which a Cu/Ti composite reinforcement is employed as in JP-A No. 2008-300337, since the Ti reacts with the Cu to form a Cu—Ti based compound, the Cu/Ti composite reinforcement is weak to bending strain, and has insufficient performance as a high strength material. Moreover, when avoiding the formation of the metal compound, an Nb layer needs to be formed between the Ti and Cu, and there is a problem in that the process is more complicated in which a three-layered reinforcement element wire is formed.
In the assembly method of superconducting wire and stainless steel as described in JP-A No. H05-217433, the performance as a stabilizer is lowered due to the high resistance value of the stainless steel, with problems with stability during conducting.